One of the key events during normal haemostasis is the conversion of the zymogen factor X (FX) into its enzymatically active form FXa, a process which subsequently leads to prothrombin activation and clot formation. In vivo FX activation is initially achieved through the tissue factor/factor VIIa pathway. The tissue factor/factor VIIa complex is readily inhibited by the tissue factor pathway inhibitor. The activation of the major part of FX subsequently occurs via the intrinsic coagulation pathway and requires formation of the intrinsic factor X-activating complex. This complex consists of the coagulation factors Factor IXa (FIXa) and Factor VIIIa (F VIIIa) assembled on a phospholipid surface in the presence of Ca2+ ions. The intrinsic factor X-activating complex produces FXa at a level that enables the formation of a stable clot.
FVIIIa functions as an activator of factor IXa (F IXa), which increases the rate of FXa formation approximately 200,000-fold (van Dieijen et al., (1981) J. Biol. Chem. 256:3433-3442). The exact mechanism by which FVIIIa enhances the catalytic activity of FIXa towards FX is still unknown. Natural occurring mutants, site-directed mutagenesis, as well as the analysis of similar cofactor/enzyme complexes, like the prothrombinase complex, suggest that there are at least two contact regions between FVIIIa and FIXa. Both regions play an important role in enhancing FIXa enzymatic activity. It is generally believed that FVIII has three functions within the intrinsic factor X-activating complex: (i) FVIII stabilizes a conformation of FIXa which has increased protease activity towards FX, (ii) FVIIIa acts as a receptor for FIXa on activated platelets which in vivo provide the procoagulant phospholipid surface and (iii) recent data indicates that FVIIIa orients the cleavage sites in FX towards the active site of FIXa.
The crucial role of FVIII in haemostasis is demonstrated by hemophilia A, a severe X-chromosome-linked recessive bleeding disorder, which is characterized by the absence of coagulation factor FVIII activity. Patients with haemophilia A are treated by administering FVIII to a patient via intravenous injection of either plasma-derived or recombinant FVIII. Although such methods are efficient, they suffer from several drawbacks. First, the relatively short half-life of FVIII means that it is necessary to administer high doses of FVIII two to three times a week. Second, FVIII production is very expensive; consequently, it is available primarily only in the industrialized world. Finally, approximately 30% of severely affected patients develop antibodies that inhibit FVIII activity (“hemophilia inhibitor patients” or simply “inhibitor patients”), which is a serious and life threatening complication.
In view of the key role that FVIII plays in haemostasis coupled with the foregoing shortcomings in delivering it to patients with blood coagulation disorders, there remains a significant need for compounds that have activities similar to FVIII but which overcome some of its limitations.